Just received email warning as follows:
Friday 13 October 2017 midday to Sunday 15 October 2017 midday
Due to a major upgrade to the National Insurance and PAYE Service (NPS) the majority of services in the Personal Tax Account (PTA) will be unavailable from midday Friday 13 October 2017 to midday Sunday 15 October 2017.
This affects PAYE and self-assessment.
HMRC apologises for the inconvenience.